The present invention relates to electronic signal conversion, and more specifically, to the conversion of single-ended signals to differential signals and differential signals to single-ended signals using a common mode shunt.
The advantages of differential circuits are well known especially when incorporated into RF integrated circuits. Often the input and/or output signal needs to be single-ended. The use of differential circuits in this case requires the use of some single-ended to differential conversion. The differential transistor pair is a circuit often used to convert single-ended inputs to differential signals. This circuit does not work well if the current source biasing the differential pair does not have a high impedance at high frequencies. A balanced/unbalanced (BALUN) impedance matching network is a type of high frequency transformer that can be used for differential/single-ended conversions. The BALUN, however, is moderately expensive and bulky. Various transmission line combiners are also known.
A single-ended to differential converter in accordance with an embodiment of the present invention includes a common mode shunt having a common mode junction and a load element. The common mode shunt includes first and second reactive elements each having first ends coupled together at the common mode junction. The second end of the first reactive element receives a single-ended input signal referenced to a reference signal. The second ends of both of the first and second reactive elements develop first and second polarities, respectively, of a differential output signal. The common mode shunt further includes a third reactive element having a first end coupled to the common mode junction and a second end referenced to the reference signal. The load element has a first end coupled to the second end of the second reactive element and a second end referenced to the reference signal.
The reference signal may be ground or any other suitable common signal serving as a voltage reference for the single-ended input signal. In one embodiment, for example, the load element and the third reactive elements are both coupled to ground. An input source provides the single-ended input signal to the second end of the first reactive element. In one embodiment, the input source includes a series-coupled input impedance element. The load element and the series-coupled input impedance element may be resistors having approximately the same resistance. The first and second reactive elements of the common mode shunt may have approximately the same impedance. In a particular embodiment, the first and second reactive elements are inductors and the third reactive element is a capacitor. In this embodiment, the inductors may each have approximately the same inductance. In the same or alternative embodiment, the first and second reactive elements have an impedance of approximately Z, and the third reactive element has an impedance of approximately xe2x88x920.5Z. This is easily achieved using inductive and capacitive elements.
A differential to single-ended converter in accordance with an embodiment of the present invention also includes a common mode shunt and a load element. In fact, the configuration may be substantially identical, except that the second ends of the first and second reactive elements receive first and second polarities, respectively, of a differential input signal and the second end of the second reactive element develops a single-ended output signal.
Again, the reference signal may be ground. The first and second reactive elements of the common mode shunt may have approximately the same impedance. The first and second reactive elements may be inductors and the third reactive element may be a capacitor. The inductors may each have approximately the same inductance. The differential input signal may be current-based polarity signals, and may be developed by a differential pair of transistors. A fourth reactive element having an impedance of approximately xe2x88x922Z may be coupled between the second ends of the first and second reactive elements, where the first and second reactive elements each have an impedance of approximately Z.